M. H. Garrett

High beam-coupling gain and deep- and shallow-trap effects in cobalt-doped barium titanate, BaTiO 3 :Co

A series of cobalt-doped barium titanate (BaTiO3) crystals were grown; these crystals have the highest beam-coupling gain reported to date, 7.9 cm−1, measured by using the smallest electro-optic coefficient of BaTiO3. The intensity dependence of the absorption and the gain indicate that there are at least two active photorefractive species. These samples show light-induced absorption and negative absorptive coupling, which is indicative of crystals with deep and shallow traps. However, doping with cobalt diminishes the intensity dependence of the electro-optic gain and increases the electro-optic and absorptive coupling, which indicates that cobalt is a deep-level photorefractive dopant. The higher gain is attributed to an increase of the total effective trap density and to an increase in the deep- and shallow-trap intensity-dependent factor η(I), with higher cobalt doping.

High photorefractive sensitivity in an n-type 45°-cut BaTiO 3 crystal

We report the beam-coupling properties of a cobalt-doped oxygen-reduced n-type barium titanate crystal in the 0° and 45° crystallographic orientations. Oxygen reduction improved the response time of the 0°-cut crystal by a factor of ~4.5 without diminishing the beam-coupling gain. The 45°-cut crystal has a peak gain of ~38.7 cm−1, a response time of ~21 ms, and a photorefractive sensitivity of 3.44 cm3/kJ. We infer from response time measurements an equivalent percentage change in the dc dielectric constant and the mobility with respect to crystallographic orientation.

A method for poling barium titanate, BaTiO3

Relatively large samples of single crystal barium titanate, BaTiO3, are needed for current photonic applications including for example; optical memories, associative memories and neural networks. The process of producing single-domain crystals called poling usually involves uniaxial pressing to remove orthogonal or 90° domains and then electric field poling to remove antiparallel or 180deg; domains. A method for poling BaTiO3 that significantly reduces the processing time for single-domain sample preparation is described herein. The technique involves the removal, by etching, of surface stresses that mechanically restrict or pin the 90° domains that inhibit their elimination. In this “stress free” state, large volume samples (10 × 10 × 5 mm) are easily electrically poled into single crystals without uniaxial pressing.

Nd:YAG master-oscillator power amplifier with a rhodium-doped BaTiO(3) self-pumped phase-conjugate mirror.

We investigate Nd:YAG master-oscillator power amplifier arrangements with an infrared-sensitive rhodium-doped BaTiO3 photorefractive phase-conjugate mirror. In these setups the phase-conjugate mirror is simple to implement, compact, passive, and completely self-contained. We demonstrate a reasonable extraction efficiency (∼50%), response times in the range of several minutes with our experimental conditions and an excellent ability of the phase-conjugate mirror to compensate for severe phase aberrations.

Self-pumped phase conjugation in rhodium-doped BaTiO 3 with 1.06-µm nanosecond pulses

Efficient internal-loop self-pumped phase conjugation is demonstrated in a photorefractive rhodium-doped barium titanate crystal by repetitively pulsed nanosecond pulses from a Q-switched Nd:YAG laser operating at 1.06 microm . Phase-conjugate reflectivities as high as 32% are obtained with a response time of ~5 min at a repetition rate of 30 Hz. The ability of the phase conjugator to correct severe phase distortions is also demonstrated.

Properties of photorefractive nonstoichiometric bismuth silicon oxide, BixSiO 1.5 x +2

Single crystals of bismuth silicon oxide (BSO) were Czochralski grown nonstoichiometrically from high-purity starting materials. Several series of three crystals were grown from nonstoichiometric melts starting with a bismuth oxide–rich melt. The intrinsic absorption shoulder of BSO was modified in amplitude with growth from nonstoichiometric melts. Two-beam coupling was used to determine photorefractive material parameters of the nonstoichiometric crystals. The data were analyzed with the two-beam coupling, two-carrier single multivalent ion model of the photorefractive effect. Parameters determined include the effective trap density, the hole–electron competition parameter, and the ratio of hole-to-electron absorption. Higher absorption correspond to a higher effective trap density and gain coefficient. The two-beam coupling gains, measured in the diffusion regime, are believed to be the largest reported for BSO, 0.70 cm−1, and are correlated to nonstoichiometric-growth-induced changes in the native defect concentrations.

Spatial beam cleanup of a Nd:YAG laser operating at 1.06 µm with two-wave mixing in Rh:BaTiO 3

Spatial beam cleanup of a distorted cw Nd:YAG laser operating at lambda = 1.06 mum is demonstrated with two-wave mixing in an infrared-sensitive rhodium-doped barium titanate crystal. Because of the high coupling gain of our crystal, high efficiencies are achieved in agreement with the standard photorefractive model.

Intensity dependent absorption/transparency of a reducing BaTiO3

We report intensity dependent absorption and transparency as a function of wavelength for barium titanate. The BaTiO3 crystal examined has an as‐grown, light‐blue color due to an absorption centered at 690 nm, and when reduced in a partial pressure of 10−15 atm of oxygen it has a yellow‐orange color due to an absorption centered at 470 nm. Both energy levels are active in the reduced sample as revealed in the spectrum of the intensity dependent changes in absorption.

Light-induced dark decay and sublinear intensity dependence of the response time in cobalt-doped barium titanate

We report how the photorefractive properties of cobalt-doped barium titanate (BaTiO3) are influenced by deep and shallow traps. The light-induced dark decay of the space-charge field has two distinct time constants. The faster decay is due to the thermalization of charges from shallow traps, whereas the longer decay is that from deep traps formed when BaTiO3 is doped with cobalt. This dopant substantially decreases the dark conductivity of BaTiO3 and thus increases the dark storage time. The power coefficient of the sublinear intensity dependence of the photorefractive response time progressively increases with the cobalt-dopant concentration. Apparently cobalt doping of the deep level eliminates the effect from the shallow level in BaTiO3. Although cobalt-doped BaTiO3 has an increased effective trap density and thus an increased electrooptic gain, the response time and absorption are also increased, which thus decreases the photorefractive sensitivity.

Shallow‐trap‐induced positive absorptive two‐beam coupling ‘‘gain’’ and light‐induced transparency in nominally undoped barium titanate

We use the asymmetry of beam coupling with respect to the orientation of the polar axis in a nominally undoped barium titanate crystal to determine the electro‐optic and absorptive ‘‘gain’’ in the usual beam‐coupling geometry. For small grating wave vectors, the electro‐optic coupling vanishes but the absorptive coupling remains finite and positive. Positive absorptive coupling at small grating wave vectors is correlated with the light‐induced transparency of the crystal described herein. The intensity and grating wave vector dependence of the electro‐optic and absorptive coupling, and the light‐induced transparency are consistent with a model incorporating deep and shallow levels.